Typically in vehicles such as automobiles, many electronic control devices are installed. A leading example of such electronic control devices is an electronic control device that forms a part of a navigation system, and other examples are electronic control devices for use in controlling a variety of in-vehicle devices (e.g., the engine and the brakes) with microcomputers or for use in controlling condition display devices adapted to display a variety of vehicle conditions (e.g., the meters). In such vehicles, these electronic control devices are electronically connected with one another by communication lines to form a vehicle network. Through such vehicle network, a variety of vehicle data is transmitted and received between the electronic control devices.
As disclosed in Patent Document 1, microcomputers have been proposed that is switched to a power saving mode (i.e., a mode during which power consumption is reduced by stopping the oscillation of the clock signal while maintaining the internal conditions) when there is no need for microcomputers to conduct any processing (e.g., when the vehicle is stopped). In such electronic control device having microcomputers switchable to a power saving mode, the microcomputers are switched between the power saving mode and the normal power mode in accordance with the vehicle conditions, such that the power consumed in the electronic control device is reducible.